Sorosilicate Mineral Hemimorphite

Associated Smithsonian Expert: Jeffrey E. Post, Ph.D.

Jeffrey Post

Photograph by Cara Santelli, Smithsonian Institution

Dr. Jeffrey Post is the curator of the National Gem and Mineral Collection at the Smithsonian National Museum of Natural History. As far back as he can remember in childhood, Post collected rocks and fossils around his home near Madison, Wis. The symmetry of mineral crystals fascinated him, and experiments with a large chemistry set helped develop his interest in science. He earned a Ph.D. from Arizona State University in 1981 and joined the Smithsonian in 1984. Post’s research projects include the physical and chemical properties of fine-grained, environmentally significant minerals such as clays, manganese oxides, and iron oxides. He also uses powerful X-ray beams at the National Synchrotron Light Source at Brookhaven National Laboratory (Upton, N.Y.) to study the crystal structures of these minerals. With his Smithsonian colleagues, Post is always seeking new gem and mineral acquisitions for the Smithsonian. He analyzes specimens to resolve curatorial questions, oversees loans of Smithsonian gems to other museums, supervises the team that is building the collection website, meets with donors, and answers public inquiries about the Smithsonian mineral collection.

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Foliated mica (variety: muscovite) with an embedded hexagonal crystal of quartz
Photo courtesy of iRocks.com/The Arkenstone

Crystal Shapes and Crystal Habits

Inside a mineral, atoms arrange themselves into a specific, repeating pattern called a crystal lattice or crystal structure. The smallest three-dimensional arrangement within the lattice is called a "unit cell," which is duplicated over and over again symmetrically. At the level of the everyday world, minerals that are growing without outside interference tend to form crystals that resemble their underlying crystal structures. Scientists call that kind of general, typical appearance a "crystal habit." Of course, conditions that existed during a mineral's formation or crystal growth may change its habit, but geologists still find this attribute to be a useful tool for identifying minerals. Scientists use more than three dozen adjectives to describe crystal habits. For example, natrolite and rutile can be acicular, or needlelike; quartz often forms hexagonal prisms; pyrite and halite typically crystallize as cubes; and mica is foliated or lamellar (layered).

Bingham Copper Mine, Utah, USA
Photo by Marli Bryant Miller

About Ores and Mining

Ores are rocks that contain minerals rich in elements that are valuable to human society. Almost all the metals we use - iron, aluminum, lead, copper, zinc, uranium, and others - come from ores. To form ores, elements and compounds must be concentrated via one of several processes. In hydrothermal processes, hot water seeping through the ground may concentrate metal-rich minerals into veins. Dense minerals from which we get metals such as platinum, nickel, and chromium crystallize and settle out of some types of magma (molten rock) underground. Much of the iron we use comes from banded iron formations: rock built up from layers of sediments containing iron oxides. Other minerals become concentrated through erosion, transport, and deposition of small rock grains. Mining began in ancient Egypt, Greece, and Rome, followed by a steady stream of technological improvements during the medieval and early modern eras. Today miners use specialized heavy equipment for both surface and underground mining.

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